Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a food grade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase in the future.
Several chemical synthetic methods are known; however, these methods of producing butanol use starting materials derived from petrochemicals and are generally expensive and are not environmentally friendly. Several methods of producing butanol by fermentation are also known, for example the ABE process which is the fermentive process producing a mixture of acetone, 1-butanol, and ethanol. Acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum is one of the oldest known industrial fermentations; as are also the pathways and genes responsible for the production of these solvents. Production of 1-butanol by the ABE process is limited by the toxic effect of the 1-butanol on Clostridium acetobutylicum. In situ extractive fermentation methods using specific extractants which are nontoxic to the bacterium have been reported to enhance the production of 1-butanol by fermentation using Clostridium acetobutylicum (see, for example, Roffler et al., Biotechnol. Bioeng. 31:135-143, 1998; Roffler et al., Bioprocess Engineering 2:1-12, 1987; and Evans et al., Appl. Environ. Microbiol. 54:1662-1667, 1988).
In contrast to the native Clostridium acetobutylicum described above, recombinant microbial production hosts expressing 1-butanol, 2-butanol, and isobutanol biosynthetic pathways have also been described. These recombinant hosts have the potential of producing butanol in higher yields compared to the ABE process because they do not produce byproducts such as acetone and ethanol. With these recombinant hosts, the biological production of butanol appears to be limited by the butanol toxicity thresholds of the host microorganism used in the fermentation. U.S. Patent Publication No. 20090305370 discloses a method of making butanol from at least on fermentable carbon source that overcomes the issues of toxicity resulting in an increase in the effective titer, the effective rate, and the effective yield of butanol production by fermentation utilizing a recombinant microbial host wherein the butanol is extracted into specific organic extractants during fermentation.
Improved methods for producing and recovering butanol from a fermentation medium are continually sought. Lower cost processes and improvements to process operability are also desired. Identification of improved extractants for use with fermentation media, such as extractants exhibiting higher partition coefficients, lower viscosity, lower density, commercially useful boiling points, and sufficient microbial biocompatibility, is a continual need. Additionally, extractants that are selective for butanol over water provide certain advantages. By way of an example, an extractant that is selective for butanol over water can reduce the energy needs for a butanol fermentation process. The reduction in total energy needed to strip the butanol from the extractant can be due to the reduction in water associated with the extractant, as the energy required to strip butanol from the extractant is directly related to the amount of water present in the extractant.
The present invention satisfies the need to provide methods for recovering butanol from a fermentation medium by contacting the fermentation medium with an organic extractant composition comprising a dry solvent, wherein the dry solvent selectively extracts butanol from the fermentation medium.